Often during electrical storms or other types of electrical interference, including line faults, circuit breakers connecting sections of transmission lines open. After the circuit breakers open, in order to restore power it is necessary to close the circuit breakers. Since alternating current is present on both sides of the open circuit breaker, it is essential that the circuit breaker only be closed when the potential difference is at a minimum or a zero point across the circuit breaker. The potential difference is equal to the potential difference between respective R.M.S. voltages, plus a reactive potential difference caused by a difference of phase of the voltages at the closing time.
A number of devices are presently on the market, which are used for checking the phase-synchronization between a pair of alternating currents to allow circuit breaker closing to occur without immediately reopening the circuit breaker or causing the disturbances to the power network. The prior art devices, however, could be better. For instance, a fault is the fact that the mechanical devices do not allow uniform and consistent breaker closing over a range of temperatures and voltages. The mechanical devices are inaccurate at relatively high or low temperatures. The inaccuracy of the mechanical devices, therefore, may cause an excess power flow when breaker closing is attempted. Alternatively the inaccuracy may block closing when conditions actually are within limits.
The phase angle and closing time function are not independent in the prior art devices as is shown in U.S. Pat. No. 3,491,248 to Beckwith, et. al. and U.S. Pat. No. 3,562,545 to Rubner, et. al. Thus, breaker closing is effected when it is not wanted. Many prior art synchronizing check relays do not have any provision for measuring the R.M.S. voltage difference across the breaker. Thus, the prior art devices may attempt to close a breaker across which there is phase synchronization, but a large R.M.S. potential difference.
What is needed, then, is an accurate temperature independent solid state synchronizing check relay. The synchronizing check relay should provide a linear, or flat breaker closing response over a wide range of temperatures and voltages. The phase angle response and time response are mutually independent of temperature and voltage.